This isn't actually only a Textastic-bug, since the same thing happens in some other Markdown-implementations (Daily One, Byword). In Texstastic the same thing also happens when using MathML in HTML.
Something like this (and more complex MathML) looks good in HTML-preview iOS 6:
<title>Trigging MathML in HTML iOS bug</title>
but it isn't possible to Send As Message. The send email dialog just disapears and goes back to editor mode. It works on my iPad with iOS 5 (but the preview doesn't look as good).
Customer support service by UserEcho